Yeah, there seem to be a lot of glitches in the upload process with the current firmware (bootloader)/IDE. I've had it literally crash my computer twice by rapidly inserting/holding/removing devices during the reboot cycles. I've learned one important lesson: don't hold "Reset" - ever. Common to halt a runaway program until I can reprogram it (without device-cycling the USB connection with a plug/unplug), but it's a reliable way to totally hang the USB bus on the Leo.
I found that the most reliable way to upload is to make sure the program IS RUNNING when it goes to upload. It has no fall-backs or intelligent detection, it seems, for checking if the board is already waiting at the bootloader, so when the LED is "pulsing" and it tries to upload, it just goes "herrr, dohh... timeout", starts running the program while avrdude is still trying to contact the board (or Arduino is trying to find the port), and it wonders why it can't find the board.
Beta blues, I guess. No hard feelings. I'm having tons of luls with the best $20 I'd ever spent 